Re: [PATCH v2] cpufreq: schedutil: Move max CPU capacity to sugov_policy

From: Russell Haley
Date: Mon Oct 03 2022 - 04:57:32 EST

>We can do that since all CPUs in the same frequency domain have the
>same max capacity

Do they? In the Intel Alder Lake datasheet [1], it says that a single
power rail supplies all IA ("Intel Architecture") cores, which includes
both P cores and E cores.

I don't have anything that new, but on Haswell and Skylake, despite the
fact that each CPU has a separate policy that lists only itself in
affected_cpus, with the userspace governor I find that every core runs
at the the highest frequency set among all cores. For clarity:

$ grep . cpufreq/policy*/scaling_{setspeed,cur_freq}

It seems that these cores are in the same frequency domain, even if
cpufreq doesn't know about it. I don't know if this affects the behavior
of the governors in any way, but it might be a bug in intel_pstate that
could one day be fixed. If it is, then any heterogeneous-uarch chips
with both CPU types sharing a voltage rail would have CPUs with
different max capacity in the same frequency domain.

This might present a problem for any future attempt to harmonize
treatment of big.LITTLE between ARM and x86.